This invention relates to a method for producing a retardation film and more particularly it relates to a method for producing a retardation film in which the retardation has desired wavelength dispersion characteristics (wavelength dependence) with good productivity. The retardation film is suitably used for, for example, optical devices such as a liquid crystal display device, an optical pickup used in recorders or an optical recording medium, a light-emitting device, an optical arithmetic device, an optical communication device or a touch panel.
Retardation films are generally used in display devices such as a liquid display device and have functions such as color compensation, widening of the viewing angle and antireflection. Thermoplastic polymers such as polycarbonates obtained by polycondensing bisphenol A, polyether sulfones, polysulfones, polyvinyl alcohol or norbornene resins are generally used as a material for the retardation films.
The retardation films are usually used in, for example, a liquid crystal display device of a super twisted nematic (STN) mode for the purpose of color compensation or widening the viewing angle. As to a method for producing the retardation films, the followings are known as producing methods by mixing two polymers.
JP-A No. 4-194902 (1992) (hereinafter JP-A means Japanese Unexamined Patent Publication) describes a method for producing retardation films by mixing a polymer exhibiting a positive birefringence with a polymer exhibiting a negative birefringence. Specifically, there are respective descriptions of the retardation films each having a little viewing angle dependence and obtained by uniaxially stretching a film formed by mixing poly(2,6-dimethyl-1,4-phenylene oxide) with polystyrene or by mixing polyvinyl chloride with polymethyl methacrylate.
Japanese Patent Publication No. 2780190 describes a retardation film prepared by uniaxially stretching a film of a mixture of at least two kinds of polymers or a film of a copolymer, wherein a method for combining polymers different in positivity and negativity of photoelastic constant is disclosed. There is a specific description of polystyrene mixed with polypropylene in a ratio of 1:2.6.
JP-A No. 6-174922 (1994) describes a retardation film formed from a composition obtained by mixing two polycarbonate resins having photoelastic constants satisfying a specific relationship. Specifically, there is a description of a bisphenol A type polycarbonate resin mixed with a polycarbonate resin prepared from a raw material consisting essentially of a bispenol A derivative having two methyl groups of the central carbon in bisphenol A substituted with phenyl groups and containing a small amount of bisphenol A.
In a reflective type liquid crystal display device, especially a reflective type liquid crystal display device using only one polarizing plate, there is a case where the device is optically prepared so that the retardation film is combined with the polarizing plate to develop functions so as to produce circularly polarized light.
In such a liquid crystal display device, an optical design of the retardation film is usually prepared in order to optimize optical characteristics as the whole display device. The wavelength dispersion characteristic of the retardation which is one of optical characteristics, however, is roughly determined by the material constituting the retardation film. Furthermore, practically usable materials are limited. In general, since polymers are mutually poor in compatibility, phase separation is caused when the polymers are mixed. When the resulting mixture is optically observed, the haze is resultantly high and the mixture is unsuitable for the retardation film. The combination of mutually compatible polymers is extremely rare such as that described in the above J-PA No. 4-194902 (1992).
Therefore, in the present situation, it is difficult to prepare the optical design so that the wavelength dispersion of the retardation of a retardation film matches with the wavelength dispersion of the retardation of a liquid crystal cell because the kinds of practically usable polymeric materials are limited. There are problems that manufacturers of the retardation films should hold a great many materials to consider the formation of the materials into films in order to provide various retardation films having the wavelength dispersion characteristics of the retardation required by many liquid crystal display device producers. Further, in the combination of the compatible polymers as described above of polymers of different kinds, the kinds are limited and there are problems about heat durability or productivity.
JP-A No. 6-230368 (1994) describes a retardation film having a birefringence of zero at a certain wavelength of visible light, wherein there is a description of the retardation film obtained by a method for laminating two retardation films different in wavelength dependence of the retardation or different in positivity and negativity of the photoelastic constant in a certain direction, or by a method for stretching a mixture of plural polymers or copolymers.
However, no description is specifically given of what kind of polymers or copolymers can be used as to a method for stretching the mixture of the plural polymers or copolymers at all.
The following techniques of laminating two films are known as methods for producing a retardation film having the controlled wavelength dispersion of the retardation.
JP-A No. 5-27119 (1993) describes the production of a retardation film by laminating two birefringent films having a specific retardation.
Japanese Patent Publication No. 2609139 describes the production of a retardation film by laminating two or more specific birefringent films different in wavelength dependence of the retardation at a specific angle.
In these cases, steps of laminating plural retardation films or adjusting the angle of lamination are required and there are problems in productivity because the plural retardation films are used. Since the thickness of the whole retardation films is increased, there are problems that the light transmittance is lowered and a display device becomes thick or dark when the retardation films are assembled therein.
A main object of the present invention is to provide a method for producing a retardation film capable of controlling the wavelength dispersion characteristics of the retardation, readily and at a high level.
Another object of the present invention is to provide a method for producing a retardation film quickly responsive to requirements for the various wavelength dispersion characteristics from customers.
A further object of the present invention is to provide a method for producing one retardation film formed of the same kind of polymer, readily and with good productivity.
The present inventors have searched for a method for producing a retardation film, readily and under good control, wherein the retardation has desired wavelength dispersion characteristics, by placing emphasis on a method for mixing two polymers, in order to solve the above problems. Especially, the inventors have made intensive studies on polymeric materials for the retardation film. The present inventors considered that the compatibility among polymers used is extremely important while taking optical characteristics such as high transparency or high optical uniformity required by the retardation film, mechanical characteristics, handleability and film formability at ultrahigh levels into consideration. As a result, the inventors have found a method for efficiently producing the above objective retardation film by combining at least two kinds of copolymers having the same repeating units and different in copolymerization composition and by controlling the mixing ratio thereof. Thereby, the present invention has been accomplished.
Namely, the present invention is a method for producing a retardation film, which comprises the steps of mixing mutually compatible polymers A and B which satisfy the following conditions (1) and (2), and of forming the resulting mixture into a film, wherein the mixing ratio is adjusted so that the film has desired wavelength dispersion characteristics of retardation:
(1) the polymer A is a copolymer comprising repeating units a and b, and
(2) the polymer B is a copolymer comprising the repeating units a and b and is different from the polymer A in copolymerization composition.